This invention generally relates to modulating a carrier wave for signal transmission and, more particularly, to phase/frequency shifting a light beam for use in optical signal transmission. This invention is the result of a contract with the Department of Energy (Contract No. W-7405-ENG-36).
Electromagnetic radiation in the optical frequency range is being increasingly used as a carrier signal for information transmission. Electromagnetic carrier signals may typically be amplitude modulated or frequency modulated to transmit information with the carrier. Where the carrier is a light beam, various electrooptic devices have been used to modulate the carrier signal. It will be understood that frequency modulation involves a frequency change or phase shift in the carrier signal.
One conventional device for intensity, or amplitude, modulating a light beam is a Pockels cell. Crystals are provided which undergo a change in the refractive characteristics in an applied electric field. A relatively high voltage driver is required to activate the crystals. Further, carrier intensity is limited by temperature sensitivity of the crystals which exhibit the Pockels effect and the carrier signal may be attenuated while traversing the modulator. Further, the crystal response times are limited, thereby limiting the information content which can be transmitted and the rise time pulse capability.
Yet another modulating device for light beams uses the Faraday effect. In this effect the polarization of a light beam can be rotated when it passes through material in the direction of an applied magnetic field. With the magnetic field the operating frequencies and pulse rise times are limited by inductive affects. Again, the carrier light beam is limited in power and information-carrying capability.
These and other problems of the prior art are addressed by the present device and an improved light beam carrier phase shifter is provided for signal carrier transmission. Accordingly, one object of the present invention is to obtain a wide range of signal modulation capability.
Another object of the present invention is to decrease the rise time for a modulating pulse.
Yet another object is to enable increased power capability for the carrier signal.
One other object is to enable relatively low-voltage drivers to be used in the modulating system.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.